Traditionally optical colour scanning of physical objects is performed by recording three or more colour intensities using an image sensor e.g. a red colour intensity, a green colour intensity and blue colour intensity, for a plurality of lines of the physical object. By combining the three or more recorded colours, lines comprising full colour pixel may be formed. Typically image sensors such as charged coupled device (CCD) or contact image sensors (CIS) are used.
As a result of the optical systems used in commercially available image scanners, only a single colour of a single point on the physical object may be recorded at a time. Thus a full colour line comprises image data recorded at different points in time, that may be spatially misaligned. This may create colour artefacts as a result of misalignment errors. This is especially a problem when scanning an object comprising high frequency information. A common resulting artefact is the introduction of strong colours in a scanned image, not present on the original physical object, this artefact may be known as colour fringing.
US2010309533A discloses a method and apparatus for scanning documents using a contact image scanner. The imaging apparatus comprises a light source for illuminating the document, a plurality of pixel sensors for generating a signal in response to light incident thereon, a lens for focusing light from the light source reflected from the document onto the plurality of pixel sensors, a colour matrix filter disposed between the pixel sensors and the light reflected from the document, and a circuit for processing the signal from the plurality of pixel sensors to generate the digital image of the document. The method comprises the steps of providing a plurality of pixel sensors for generating a signal in response to light incident thereon, projecting light from a light source onto the document in a sequential fashion along the length dimension thereof, reflecting the light projected onto the document onto the plurality of pixel sensors, filtering the light from the light source with a colour matrix filter, and processing the signal from the plurality of pixel sensors to generate the digital image of the document.
The method disclosed in US2010309533A may remove some artefacts however a new type of artefacts is introduced in the individual lines, due to the measurement setup. The introduction of a colour matrix filter will additionally increase the complexity and the cost of an image scanner made in accordance with the principles disclosed in US2010309533A
Thus, it remains a problem to improve the image quality of colour scans in a simple and reliable manner.
Additionally it remains a problem to provide a method and/or scanner for reducing image sensor dependent artefacts when scanning large format documents using a plurality of image sensors.